The problem is that Tkinter (at least for Windows) is really based on 90s widgets. You could set PySimpleGUI's theme to "SystemDefaultForReal" but it will still

The problem is that Tkinter (at least for Windows) is really based on 90s widgets. You could set PySimpleGUI's theme to "SystemDefaultForReal" but it will still look out-of-place because the native Windows widgets has that modern look while Tkinter has never been updated at all.